A primary method for manufacture of polyesters which is well known in the art, involves a two step process; a melt polymerization step which produces an intermediate molecular weight polymer, followed by solid state polymerization leading to high molecular weight polyester. Generally, two methods are employed in commercial polyester production for the melt phase step. For example, poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) can be made by the reaction of 2,6-naphthalenedicarboxylic acid with excess ethylene glycol (EG). With this method, a prepolymer is first prepared which is advanced with heat and vacuum treatment to a sufficient molecular weight for solid state polymerization. A second method involves an ester exchange reaction between dimethyl-2,6-naphthalene dicarboxylate (DMN) and excess EG to give bis-hydroxyethylenenaphthalate (BHEN) and low molecular weight oligomers, which are polymerized with heat and vacuum treatment to high polymer. For both methods, catalysts are employed in the polymerization step. Commonly used polymerization catalysts are antimony, titanium, tin and germanium for both methods. Common catalytic species employed for ester exchange include zinc, manganese, calcium, and lithium.
The ester exchange catalysts listed are limited in utility when water is present in EG or the starting EG/DMN slurry. A marked reduction in catalyst activity takes place when a small quantity of water is present in the reaction mixture at the start of ester exchange. Catalyst deactivation in this crucial step results in poor conversion of DMN to BHEN. Generally, it has been found that conversions of 96 percent or greater are required for good polymerization rates. High methyl end content, which is a measure of unreacted DMN ester groups remaining in the ester exchange product, lowers melt phase and solid state polymerization rates. In PEN applications where high molecular weight is required, such as tire cord fiber, high methyl ends in the precursor polymer often times prohibits reaching the desired molecular weight during solid state polymerization.
Water in the ester exchange step of melt phase polyester synthesis is well known in the art to deactivate to a greater or lesser extent catalyst activity, depending on the amount of water present in the reaction. U.S. Pat. No. 5,138,025 granted to the Amoco Corporation teaches a method whereby DMN in melt form may be treated to minimize the effect of moisture and oxygen on product quality. Poor ester exchange rate may also arise from water in EG. In a typical manufacturing process for PEN, EG collected from the previous batches (for batch operations) or in the case of a continuous manufacturing process, EG from prior line operation, is refined by distillation to remove impurities, including water, and used again in the process. For economic reasons, such refining may occur in a centralized facility, wherein EG from PEN production may be combined with EG from production of other polyesters. Often times, refined EG will have a water content greater than or equal to one weight percent. This compares to the commercial grade EG, defined here as commercially available polymer grade EG, which typically has water content of less than 0.3 weight percent. The use of commercial grade EG alone in PEN manufacturing processes, with no refined EG use, although attractive from the stand point of good ester exchange rate, is not commercially viable as a result of the expense associated with not recycling this monomer. Although spent commercial grade EG which has gone through the PEN process is distilled to produce refined EG, further distillation to remove water is both costly from the stand point of additional equipment required for further purification of EG, and time consuming.
As mentioned above, the use of commercial grade EG solely in the PEN manufacturing process is not viable due to cost. However, even if commercial grade EG were to be used to make EG/DMN slurry, there is still the potential for water take up from the surroundings. Water can come into contact with the slurry from moisture content in DMN, humidity of the surroundings, including mix tanks and monomer conveying systems, moisture in inert gas used to blanket the reaction, among other sources.
Typical prior art patents include the following.
U.S. Pat. No. 3,756,987 to Winnick discloses a process for preparing polyethylene terephthalate from a diester consisting essentially of a bis-(beta-acyloxyethyl) terephthalate in the presence of sufficient water to react with at least 25% to about 100% of the acyl moieties to liberate the corresponding lower carboxylic acid. The resulting bis-(beta-hydroxyethyl) terephthalate is then polymerized to form polyethylene terephthalate.
U.S. Pat. No. 3,936,421 to Hayashi et al. describes a process for preparing polybutylene terephthalate from terephthalic acid and 1,4-butanediol, where water is present during esterification and can be present in the 1,4-butanediol feed.
U.S. Pat. No. 4,058,507 to Omoto et al. discloses a process for preparing polyethylene terephthalate from dimethyl terephthalate and ethylene glycol, using calcium acetate and cobalt acetate as catalysts, where 0.03 to 0.2 wt. % water is present in the ester exchange reaction system.
U.S. Pat. No. 4,097,468 to James et al. describes a process for continuous esterification, where the feed material is a mixture of terephthalic acid, ethylene glycol, water, and partially esterified terephthalic acid.
U.S. Pat. No. 4,990,594 to Cooke et al. and U.S. Pat. No. 5,082,731 to Cooke et al. describe a process for producing a copolyester from the ester exchange of a lower dialkyl ester of a dicarboxylic acid and glycol and the direct esterification of a dicarboxylic acid and glycol using a catalyst system containing Mn, Li, Sb, and optionally Co, along with an agent to sequester the Mn.
U.S. Pat. No. 5,019,640 to Engel-Bader et al. discloses a process for producing polyethylene terephthalate from a lower dialkyl ester of a dicarboxylic acid and glycol, where Mn and Li are used in the ester exchange section.
U.S. Pat. No. 5,101,008 to Cooke et al. describes a process for making a copolyester from at least two lower dialkyl esters of dicarboxylic acids, wherein Mn, Li, and Sb, and optionally Co or a catalyst sequestering agent are present.
U.S. Pat. No. 5,116,938 to Engel-Bader et al. discloses a process for producing a polyester from a lower dialkyl ester of a dicarboxylic acid and glycol, wherein Mn and Li are used in the ester exchange section, and Co and Sb are used as catalysts during the polycondensation section.
U.S. Pat. No. 5,811,513 to Iwasaki et al. discloses a process for producing polyethylene naphthalate from naphthalenedicarboxylic acid and ethylene glycol in the presence of water, wherein the amount of water is more than 0.03 to 1.5 times that of the ethylene glycol, on a weight basis.
U.S. Pat. No. 5,895,807 to Galko et al. describes a process for making a random polyalkylene terephthalate/naphthalate copolymer, wherein the naphthalate-bearing feedstock is a diester and the terephthalate-bearing feedstock is a diacid.
Japanese Pat. No. 04080931 B to Kamata et al. describes a process wherein at least one water-soluble metal compound selected from Li, Ca, Mn, Co, and Zn is added to a solvent consisting of 10-95 wt. % ethylene glycol and 5-90 wt. % water. This catalyst solution is used in subsequent polymerization of terephthalic acid (or a derivative) with ethylene glycol.
Accordingly, there is a need for a process whereby refined diol component can be used in producing PEN and copolymers of PEN while maintaining good ester exchange activity, and which PEN and copolyesters of PEN are characterized by having low methyl end-group content, low diethylene glycol content, and low carboxyl end-group content.